1. Field of the Invention
The present invention relates to an integrated process and installation for the production of synthesis gas. In particular it relates to a process using two reactors producing synthesis gases containing at least hydrogen and carbon monoxide with a global hydrogen/carbon monoxide ratio between 1.8:1 and 3:1.
2. Description of the Related Art
Due to the economic benefits associated with using natural gas on certain gasfields or oilfields and recent advances in catalytic processes, a certain number of projects for converting natural gas to synthetic hydrocarbons are presently being studied. The processes used can, for example, produce synthetic fuel by the gas-to-liquid process (GTL), olefins by the gas-to-olefins process (GTO), methanol or dimethyl ether (DME). GTL processes are described in ‘Shell Middle Distillates Synthesis’ by P. Tijm et al., Alternate Energy '94 Apr. 26-29 1994.
These processes generally include three steps:                1) production of synthesis gas (mixture of hydrogen and carbon monoxide)        2) synthesis of hydrocarbon chains        3) distillation and/ or finishing and/or hydrocracking        
Most of these processes use large amounts of oxygen or oxygen enriched air to produce the synthesis gases in partial oxidation reactors using a non-catalytic or catalytic process. A suitable air separation unit for producing oxygen is described in EP-A-0982554.
The following explanation and description relates to GTL plants but applies also to other synthetic hydrocarbon plants, such as GTO, DME or methanol plants. For GTL processes, the Fischer-Tropsch reactionnCO+2nH2→(—CH2)n+nH2Orequires a stoechiometric synthesis gas make-up to be produced with a molar ratio of 2:1.Additional amounts of hydrogen are needed for the finishing and to compensate for losses in by-products and/or purge gases leading to an increased global H2/CO ratio of between 2.1:1 and 2.7:1.
Typically a non-catalytic partial oxidation POX unit, when fed with natural gas, produces a synthesis gas with an H2/CO ratio of about 1.8:1 depending on the composition of the natural gas. This ratio can vary too when other oxidants, such as steam or carbon dioxide, are sent to the unit.
The global H2/CO ratio can be reached                either by partial shift conversion of the CO produced in the POX unit as described in EP-A-0484136CO+H2O→CO2+H2        or by coproducing a second synthesis gas from a steam methane reformer unit (SMR), fed also by natural gas, the second synthesis gas having a H2/CO ratio typically between 2.7/1 and 6/1.        
Thus, the synthesis gas from the POX unit can be combined with synthesis gas from an SMR unit to produce the required global ratio.
When the POX unit is fed with heavier feed stock, such as coal, residues or intermediate by-products, the H2/CO ratio is typically lower and the above techniques must be used to balance the overall H2 requirements.
Catalytic partial oxidation processes, when fed with natural gas or other light hydrocarbon mixtures, produce synthesis gas with a higher H2/CO ratio between 2/1 and 3/1 and can be used, as stand alone processes or not, to satisfy the global ratio.
For a given size of GTL plant, using an oxygen fed reactor such as a POX unit and a process which does not use oxygen such as an SMR unit, it is an object of the present invention to optimise the size and number of the POX modules and/or SMR modules and/or ASU modules, constituting the POX unit, SMR unit and the air separation unit, using the latest technical developments for the various modules.
Particularly, in recent years, the output of air separation units has considerably increased. Modular units presently produce 3500 tonnes of oxygen per day and the module should be able to produce 6000 tonnes of oxygen per day in the near future.